This invention relates generally to nondestructive testing, and more particularly to an eddy current array probe and methods of assembling the same.
Eddy current (EC) inspection devices are used to detect abnormal indications in a conductive component being tested such as, but not limited to, gas turbine engine components. For example, known EC inspection devices may be used to detect cracks, pings, dings, raised material, and/or other surface and subsurface imperfections on a surface of the component, and/or to evaluate material properties of the component including the electrical conductivity, density, and/or degrees of heat treatment of the component.
During operation, known EC devices measure the interaction between an electromagnetic field generated by the EC device and the component being tested. For example, at least some known EC devices include a probe coil that generates a magnetic field. When the coil is positioned adjacent to a conductive component, an eddy current is generated on the surface of the component. A flaw on and/or near the surface of the component disrupts the eddy current field causing a secondary field to be produced that is received by the eddy current probe coil or by a sensor coil in the eddy current probe. The secondary field is converted to an electrical signal that may be observed on a monitor or recorded, for example, on a strip chart recorder.
In use, a substantially constant pressure is applied to the probe as the coil moves along the surface of the component being tested. The constant pressure facilitates maintaining an integrity of the signal generated by the EC probe. However, when the EC probe is not oriented substantially normal to the surface of the component being tested, a “lift-off effect” may be created.
To facilitate reducing lift-off-effects, at least one known EC probe includes a dual-coil probe, e.g. a differential probe that includes a pair of coils with an opposite polarity. Each coil in the dual-coil probe generates an electrical signal when the probe contacts a surface of the component being tested. More specifically, when the dual coil probe passes over a smooth surface of the component being tested, the signals cancel each other. However, when the dual coil probe passes over a local physical abnormality on the surface, the probe generates a signal that is proportional to the size, depth, etc., of the physical abnormality.
When a non-continuous component surface feature is inspected, such as a feature on a rotating part, known differential probes may have difficulty testing sharp curvatures, in such areas as corners and cusps. During operation, when such probes encounter a corner or cusp, the differential probe device may become skewed to the surface of the component, such that a resulting lift-off effect may cause a loss of usable data. Accordingly, known EC probes may be less effective in generating an accurate response when the EC probe is used to detect conditions on a component having complex geometries, and/or a component having irregular conditions, such as may be prevalent in components including sharp indexing or objects that extend into the path of the probe such that the probe cannot consistently remain normal to the scan surface. Known EC probes use coils as the sensing element to detect surface flaws. In order to accurately detect small surface flaws, a probe must provide a combination of high sensitivity and high spatial resolution.